1. Field of the Invention
The present invention relates to annulus pressure responsive downhole tools. Particularly, the present invention provides a low pressure responsive downhole tool with a hydraulic means for locking the tool in a chosen position during subsequent changes in well annulus pressure. 2. Description Of The Prior Art
The prior art includes a variety of downhole tools such as testing valves, circulating valves and samplers which are operated in response to a change in well annulus pressure. One particular type of annulus pressure responsive tool has previously been developed by the assignee of the present invention and is generally referred to as a low pressure responsive tool.
An example of such a low pressure responsive tool is shown in U.S. Pat. No. 4,667,743 to Ringgenberg et al. The low pressure responsive tool includes a power piston having first and second sides communicated with the well annulus through first and second pressure conducting passages defined in the tool. A retarding means, such as a metering orifice, is placed in the second pressure conducting passage for delaying communication of a change in well annulus pressure to the second side of the power piston for a sufficient time to allow a pressure differential across the power piston to move the power piston. The movement of the power piston is typically accommodated by compression of a compressible gas such as nitrogen.
It is desirable with such tools to be able to selectively lock the power piston and the associated operating element of the tool in a chosen position so as to disable them during subsequent changes in well annulus pressure
The prior art has approached this problem by providing mechanical position control devices such as a lug and slot ratchet assembly like that shown in Ringgenberg et al. U.S. Pat. 4,667,743.
One disadvantage of the use of mechanical position control schemes such as that of Ringgenberg et al. is that the power piston must move through a predetermined series of movements in order to obtain a selected position, as is determined by the various positions defined on the ratchet assembly. Also, the tool is only held in a chosen position for a predetermined number of well annulus pressure cycles